1. Field of the Invention
The present invention relates to a method and related communication device in a wireless communication system for improving Radio Resource Control (RRC) connection establishment, and more particularly, to a method and related communication device for improving re-initiation of Non Access Stratum (NAS) procedures as a result of lower layer failure, for establishing an RRC connection more efficiently.
2. Description of the Prior Art
A long-term evolution (LTE) system, initiated by the third generation partnership project (3GPP), is now being regarded as a new radio interface and radio network architecture that provides a high data rate, low latency, packet optimization, and improved system capacity and coverage. In the LTE system, an evolved universal terrestrial radio access network (E-UTRAN) includes a plurality of evolved Node-Bs (eNBs) and communicates with a plurality of mobile stations, also referred as user equipments (UEs).
In the LTE system, a UE in idle mode establishes a Radio Resource Control (RRC) connection for signalling or data transmission. The UE sends an RRC connection request message including an RRC connection establishment cause to an eNB to request to establish an RRC connection. The eNB uses the RRC connection establishment cause to prioritize the RRC connection request from the UE, e.g. at high load situations. Therefore, the UE has to include the correct RRC connection establishment cause for the eNB to prioritize the RRC connection establishment. Currently, in the LTE system, RRC connection establishment causes are defined as follows: emergency call, high priority access, Mobile Terminating (MT) access, Mobile Originating (MO) signaling and MO data.
These RRC connection establishment causes are mainly used by the network to differentiate the subsequent action upon receiving the RRC connection request message. For example, the RRC connection establishment cause can be used to perform a call admission control. On the other hand, an access barring control is performed to prioritize access of Non Access Stratum (NAS) connection request types, which are mapped to RRC connection establishment causes.
According to current 3GPP Technical Specifications, UE NAS performs a NAS procedure, such as a Tracking Area Update (TAU) procedure, an attach procedure or a service request procedure, to request UE RRC to establish an RRC connection. When a lower layer failure such as cell reselection or cell barred occurs, UE NAS re-initiates the NAS procedure to establish the RRC connection. Note that, cell barred is an event occurred when a cell is considered barred during the initiation of an RRC connection establishment. During cell barred, the UE performs access barring control function to limit the UE to perform the specific access. Current method of initiating the NAS procedure results in an aborted NAS procedure or unnecessary waiting time for RRC connection establishment, which are illustrated in FIG. 1 to FIG. 8 and are described by issues as below.
First note that, it is specified that when cell reselection occurs during RRC connection establishment, the UE informs upper layers of the failure to establish the RRC connection, upon which the procedure ends. Please refer to FIG. 1, which is a sequence diagram of a TAU procedure, for issue 1. As shown in FIG. 1, the UE performs the TAU procedure via a cell 1. NAS transmits a TAU request message to request RRC to establish an RRC connection. Before the RRC connection is established, RRC performs cell reselection to a cell 2 with the same Tracking Area (TA) as the cell 1. In this situation, RRC indicates a failure message to NAS, and NAS starts a timer T3411 accordingly. The timer T3411 is a timer starting at the TAU procedure failure or at an attach procedure failure due to lower layer failures, and currently the period of the timer T3411 is specified as 10 seconds. According to current NAS specification, NAS cannot initiate the TAU procedure until the timer T3411 expires. As a result, the UE cannot have service access, make a MO call or receive a MT call before the TAU procedure is completed, even if cell reselection is completed much earlier. A similar issue may also occur for the attach procedure.
Please refer to FIG. 2, which is a sequence diagram of a service request procedure, for issue 2. The UE receives a paging message for a MT call, and NAS sends a service request message as a paging response to request RRC to initiate an RRC connection establishment procedure. Before the RRC connection is established, RRC performs cell reselection, so as to terminate the RRC connection establishment procedure and indicate a failure message to NAS. However, NAS behavior is not specified in current specification when NAS receives the failure message from RRC during the service request procedure. As a result, NAS does not retransmit the service request message, and apparently, nothing is done. The MT call could be missed because the UE hangs up the calls due to no answering or call entering voice box.
It is specified that if a cell is considered barred during the initiation of RRC connection establishment, the UE informs upper layers of the failure to establish the RRC connection, upon which the procedure ends. Please refer to FIG. 3, which is a sequence diagram of a TAU procedure, for issue 3. The UE performs a TAU procedure via a cell with access barring for signalling is configured. RRC receives a TAU request message and draws a random value “rand” 0≦rand<1. This random value is greater than or equal to an access probability factor, so RRC considers the cell barred and starts a timer T305 accordingly. Meanwhile, RRC indicates a failure message to NAS to start the timer T3411. Note that the timer T305 is equal to (0.7+0.6×rand)×accessBarringTime, where a value of the parameter “accessBaringTime” is a time length configured in one of {4 s, 8 s, 16 s, 32 s, 64 s, 128 s, 256 s, 512 s}. As mentioned previously, the UE cannot initiate the TAU procedure until the timer T3411 expires. In other words, the UE cannot have service access, make a MO call or receive a MT call before the TAU procedure is completed, even if the timer T305 expires earlier than the timer T3411. A similar issue may also occur for an attach procedure.
Please refer to FIG. 4, which is a sequence diagram of a service request procedure, for issue 4. The UE performs the service request procedure via a cell with access barring for originating calls is configured. Please note that NAS starts a timer T3417, which is specified as 5 seconds in current specification, when sending a service request message. RRC receives the service request message and draws a random value “rand”, which is described previously, considers the cell barred and starts a timer T303 accordingly. Similar to the timer T305, the timer T303 is also equal to (0.7+0.6×rand)×accessBarringTime. Meanwhile, RRC indicates a failure message to NAS due to cell barred. After the timer T3417 expires, NAS aborts the service request procedure and does not retransmit the service request message. As a result, the UE cannot initiate the service request procedure even if the timer T303 expires earlier than the timer T3417.
From the above, FIG. 1 and FIG. 2 illustrate that cell reselection occurs during the TAU/Attach procedure and the service request procedure; and on the other hand, FIG. 3 and FIG. 4 illustrate that cell barred occurs during the TAU/Attach procedure and the service request procedure.
Please refer to FIG. 5 and FIG. 6 for issue 5. FIG. 5 is a sequence diagram of a service request procedure illustrating a collision of a MO call and a paging occurs after cell barred occurs. FIG. 6 is a sequence diagram of a service request procedure illustrating a collision of a MO call and a paging occurring after cell reselection occurs. As shown in FIG. 6, NAS sends a service request message of a MO call and starts the timer T3417. Before the RRC connection is established, RRC performs cell reselection and indicates a failure message to NAS. Next, a paging is received. It is not clearly specified in current NAS specification whether the UE can send another service request message for a different purpose, e.g. a paging response or an emergency call, when the original service request procedure is pending. As a result, the MT call could be missed because the UE does not send the service request message for a paging response. The scenario in FIG. 5 is similar to the scenario in FIG. 6, and is not repeated here.
Please refer to FIG. 7, which is a sequence diagram of a periodic TAU procedure, for issue 6. At the beginning, a periodic TAU timer expires and the UE receives a paging message. It is specified that a service request procedure can only be initiated when no UE-initiated Evolved Packet System Mobility Management (EMM) specific procedure, such as a TAU procedure, an attach procedure or a detach procedure, is ongoing. Therefore the UE performs the periodic TAU procedure first. However, the UE performs the periodic TAU procedure via a cell with access barring for signalling is configured. RRC draws a random value “rand” greater than or equal to an access probability factor so RRC considers the cell barred and starts the timer T305 accordingly. Meanwhile, RRC indicates a failure message to NAS to start a timer T3411. After the timer T3411 expires, NAS retransmits the TAU request message. It is not specified that whether the UE re-initiates the service request procedure after the TAU procedure terminates (receiving a failure message in this case). Consequently, the MT call could be missed.
Please refer to FIG. 8, which is a sequence diagram of a TAU procedure, for issue 7. The UE receives a paging message from an eNB 1 and cell reselection occurs. The UE reselects to an eNB 2 with a different tracking area and performs a TAU procedure. Since the service request procedure can only be initiated when no UE-initiated EMM specific procedure is ongoing, the UE performs the TAU procedure first and puts a paging response aside. However, the UE performs the TAU procedure via the eNB 2 with access barring for signaling is configured. In this situation, RRC draws a random value greater than or equal to access probability factor so RRC considers the cell barred and starts the timer T305 accordingly. Meanwhile, RRC indicates a failure message to NAS to start a timer T3411. NAS retransmits the TAU request message after the timer T3411 expires. Note that in this scenario, the timer T305 is longer than the timer T3411. That is, cell barred is not yet released, so the TAU request procedure cannot be completed successfully, and therefore RRC indicates the failure message again to NAS. As a result, the MT call could be missed because the UE hangs up the calls due to no answering or call entering voice box. Briefly, the UE performs the TAU procedure first and puts the paging response with MT access as the RRC connection establishment cause aside, and therefore the MT call could be missed.
In summary, the above seven issues shows that in current 3GPP specifications NAS and RRC behaviors may be time-consuming for RRC connection establishment, RRC connection establishment failure, or missed MT calls.